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Introduction and description

The plesiosauroids were sea-going reptiles with long neck and small heads filled with long sharp curved teeth. They had a flattened, barrel-shaped body and large paddle-shaped flippers. The two front flippers were always somewhat larger than the hind flippers. They ranged in size from 2.5 to 13 metres.

The plesiosauroids seem to have evolved during the Triassic, as supposed fragments occur in the Upper Keuper and Rhaetic of Europe. But it is only during the Early Jurassic that the group became well established. Beautiful skeletons are known from shallow marine deposits of Europe. Early Jurassic forms were generally quite small, about 3 meters in length, with a relatively short neck of about 32 vertebrae. As time progressed plesiosaurs developed longer necks and, by the middle Cretaceous, increased greatly in body size as well. The largest and most derived forms are known from the Late Cretaceous. These had long, snake-like necks and overall lengths of up to 13 meters. Ironically, the paddles are relatively smaller in these late forms then they were in the Jurassic types, indicating less emphasis on overall movement and more on capturing prey by stealth.

The long flexible necks enabled them to dart their small heads and catch fish in their toothy jaws. The neck became longer - and perhaps more flexible with the addition of additional, shorter vertebrae - as the group evolved through time. Plesiosauroids, unlike ichthyosaurs, used stomach stones for ballast to offset the buoyancy of their lungs. Plesiosauroids may have rested on the bottom, striking at prey with their long necks. Taylor (1981).

Plesiosaur classification has traditionally been a mess. One of the confounding factors has been the change in plesiosaur osteology during the course of development. As Brown (1981) notes:

Examples are known of plesiosauroid individuals ranging from half-grown to full-grown. Differences of osteological structure of specimens of a single species, attributable entirely to ontogeny, have in the past been misinterpreted and used to produce taxonomic divisions. This, together with misinterpretation of other characters used in taxonomy, has led to the publication of a hyperabundance of generic and specific names, and a proliferation of schemes of classification based upon these. In order to facilitate description and the study of ontogeny, specimens are here allocated to one of three categories which represent growth stages, as follows:

(i) 'Juveniles', in which the neural arches of the vertebrae are not fused to the centra;

(ii) 'Adults', in which fusion of neural arches and centra has taken place, and

(iii) 'Old adults', in which the neural arches and centra are fused, and in addition further characters of advanced ossification are found.